The present invention relates generally to memory devices, and more particularly, to a system for reference current tracking in a memory device.
In semiconductor devices, such as present FLASH memory devices, there are many instances where the current flowing through a core cell is compared to a reference current. For example, to set a core cell threshold (Vth) voltage. In this case, after the device has been programmed or erased, it is desirable to verify whether or not the program or erase functions were successful. One way to do this is to compare the core cell current to a reference current.
FIG. 1 shows a diagram of a typical circuit used to set a core cell Vth in a memory device. When the core cell current becomes equivalent to a reference current, the function (program or erase) has been successfully performed.
The reference current is required to be constant regardless of device process variations. One way to achieve this is to use a power supply external to the device. For example, an external device tester can include a power supply that provides a constant reference current. However, using an external tester device is time consuming and inefficient.
Another way to generate a constant reference current is to do it internal to the device as shown in FIG. 1. However, the internal circuit shown in FIG. 1 is also subject to process variations, and so may not provide a constant reference current. For example, if the reference current goes lower or higher, due to process variations, the core cell Vth will be set higher or lower than desired.
Therefore, it would be desirable to have a way to generate a constant reference current that does not require an external test device and does not suffer from internal process variations.
The present invention includes a system for reference current tracking in a semiconductor device. The system includes a circuit to internally generate a reference current that can be compared to a core cell current to verify device functions. The system also includes a back bias circuit that reduces the effects of process variations.
As a result, the system is operable to provide an effective constant reference current that does not require an external power source and that can be constructed from a small circuit that does not use excessive amounts of device real estate.
In one embodiment of the invention, apparatus for generating a reference current in a semiconductor device is provided. The reference current is compared to an internal device current generated by an internal device circuit to verify operation of the device. The apparatus comprises a current generator that is located within the semiconductor device and generates the reference current, and a bias generator that is coupled to the internal device circuit, wherein the bias generator generates a back bias current to offset variations to the reference current.
In another embodiment of the invention, a method for generating a reference current in a semiconductor device is provided. The reference current is compared to an internal device current generated by an internal device circuit to verify operation of the device. The method comprises steps of generating the reference current within the semiconductor device, and generating a back bias current that is coupled to the internal device circuit, wherein the back bias current offsets variations to the reference current.